Re: [ATM] Determining the focal length of a negative meniscus lens.

[Jan Bentz <jbentz@inreach.com>]

Find a known focal length positive lens that is stronger in an absolute 
sense than the negative and of more or less the same diameter.  With 
both as close together as possible, find the focal length of the 
combination.  For a lens of this size arrange them with the convex 
surface of the meniscus facing the positive element.  This is to get the 
principal planes as close together as possible and makes it easier to 
use on a nodal slide.  Use the so-called "thick lens" equation,  for the 
best accuracy,  and solve for the negative focal length. 

Jan Bentz

Thomas Janstrom wrote:

>Hi All, 
>
>OK, this one has me stumped, how do I measure the focal length of a
>negative meniscus lens? Richard mentioned using a biconvex lens of a
>known FL to "pre-collimate" the light source, but I don't see how this
>would work, unless I had a lens of the same FL to use as the one under
>test.
>
>I've gone through what books I have and they for the most part only deal
>with the testing of positive lens and they only "touch on" negative
>lenses and then they are not of the meniscus type anyway, before going
>back to positive ones. One thing I did come across was their (neg
>meniscuses) use in early achromatic objectives (read 1880's or early).
>
>I can build a test bench easily enough, but that's not really the
>problem, I just don't know how to work with negative lenses, never
>having had much to do with them in the past.
>
>I have light sources for the following lines (well near enough, at most
>they are only a few nm out), C (long by 3.6nm), e (long by 21nm),
>F'(short by 9nm), d (exact freq'), and g (short by 7.8nm). I'm hoping
>that these should give me enough info to peg the glass type used in the
>lens that I have to a reasonable degree of accuracy. For more info on
>these lenses see the thread on "reverse engineering a lens".
>
>So there we go, any help with this little conundrum would be great!
>
>Thanks all.
>
>Clear Skies, Thomas.
>www.tjanstrom.com
>www.norsewines.com.au 
>
>  
>
_______________________________________________
ATM mailing list http://www.atmlist.net/